1. Field of the Invention
This invention-relates to an electroforming apparatus for plating the surface of the object to be plated, such as an optical disc master.
2. Description of the Related Art
In the related art, an electroforming apparatus for plating an optical disc master made of glass, for instance, has a cathode part to place the optical disc master in a main tank for storing a plating solution. The cathode part has a surface on which the optical disc master is placed and is also equipped with a revolving mechanism for revolving the optical disc master placed on the cathode part surface.
A titanium basket, functioning as an anode part, is placed in the main tank face-to-face with the cathode part at a fixed distance. A plurality of nickel (Ni) balls is housed in the titanium basket.
Starting the electroforming apparatus of the above constitution permits predetermined current and voltage to be applied to a space between the titanium basket and the optical disc master for plating the surface of the optical disc master with nickel of a fixed coating thickness.
For example, the current and time for plating the surface of the optical disc master by the use of the above electroforming apparatus may be set as follows. That is, in case of subjecting an optical disc master of 200 mm in diameter, for instance, to plating of 290 to 300 xcexcm in coating thickness, the electroforming apparatus first applies current to the surface of the optical disc master to provide a current density as low as 0.2 to 0.3 A/dm2. Thereafter, the electroforming apparatus increases the current gradually in a period of 10 to 20 minutes, for instance, to increase the current density on the surface of the optical disc master up to 22 to 26 A/dm2, for instance. Then, as the result of applying the current continuously for 70 to 80 minutes while keeping the above current density, the surface of the optical disc master is subjected to plating of 300 xcexcm in coating thickness.
Incidentally, in addition to the plating process with the above electroforming apparatus, a cutting process or the like of forming pits or the like in the glass master original is required prior to the plating process for the manufacture of the optical disc master. Less process time has been required recently for cutting, permitting the cutting process to be finished in a period as short as about 30 minutes at present. Accordingly, there is a tendency toward the reduction in overall process time for the manufacture of the optical disc master. In this connection, less process time also is required for plating with the above electroforming apparatus.
In the above case, assuming that the current density on the surface of the optical disc master is increased up to 22 to 26 A/dm2, there is a need for continuous application of current for 70 to 80 minutes to subject the optical disc master of 200 mm in diameter to plating of 290 to 300 xcexcm in coating thickness.
The current density on the surface of the optical disc master has a relation to the time required, as shown in FIG. 15.
That is, an increase in current density on the surface of the optical disc master up to 50 A/dm2 is good enough to finish plating the surface of the optical disc master with a coating thickness of 300 xcexcm within 30 minutes.
However, if the current density on the surface of the optical disc master is increased up to about 50 A/dm2, a passive state of metal occurs in the titanium basket placed face-to-face with the optical disc master. The phenomenon of generation of oxygen and the chlorine gas or the like resulting from the electric discharge of the hydroxide ion and chlorine ion in preference to melting of nickel (Ni) is referred to as the passive state.
The passive state, if it occurs, reduces the pH of the plating solution in the main tank to cause decomposition of the plating solution, resulting in a problem in that the optical disc master may not be plated as desired.
On the other hand, as the result of increasing the current density on the surface of the optical disc master up to about 50 A/dm2, the temperature of the plating solution sometimes increases up to 70xc2x0 C. or more in excess of a temperature of 50 to 55xc2x0 C. in a normal condition of the plating solution, for instance, in the vicinity of the titanium basket where the plating solution is easily subjected to retention. An increase in temperature of the plating solution, as described above, causes decomposition of the plating solution as well, resulting in a problem in that the optical disc master may not be plated as desired.
It is an object of the present invention to provide an electroforming apparatus that permits plating enough to form a coating of high quality in a short period of time without causing decomposition of the plating solution.
To attain the above object, according to a first aspect of the present invention, there is provided an electroforming apparatus which comprises a container unit for storing a plating solution, a cathode part placed in the container unit and provided with an object to-be-plated, and an anode part placed in the container unit face-to-face with the cathode part, wherein a current-conductive opening of the anode part is formed to have an area larger than that of a current-conductive opening of the cathode part.
According to the above constitution, since the current-conductive opening of the anode part is formed to have an area larger than that of the current-conductive opening of the cathode part, the anode part permits less increase in current density than the cathode part, even if the current density of the cathode part is increased. Thus, no passive state occurs in the anode part, resulting in the prevention of the plating solution from being decomposed and so on.
Further, the anode part where the plating solution is easily subjected to retention permits less increase in current density than the cathode part, resulting in the prevention of the plating solution from being decomposed due to an increase in temperature as well.
Preferably, according to a second aspect of the present invention, the electroforming apparatus in the constitution as defined in the first aspect may be characterized in that the cathode part is placed in an inclined posture at a certain angle.
According to the above constitution, an inclined rotary-type electroforming apparatus having the cathode part placed in the inclined posture at the certain angle, for instance, may prevent the current density of the anode part from being increased as much as that of the cathode part, even if the current density of the cathode part is increased.
Preferably, according to a third aspect of the present invention, the electroforming apparatus in the constitution as defined in the second aspect is characterized in that the current-conductive opening of the anode part is formed to have an area two to three times or more as large as an area of the current-conductive opening of the cathode part.
According to the above constitution, since the current-conductive opening of the anode part is formed to have an area two to three times as large as the area of the current-conductive opening of the cathode part the current density of the anode part is limited to half or less of the current density of the cathode part, even if the current density of the cathode part is increased in particular. Thus, the passive state may be further prevented from occurring in the anode part, resulting in the effective prevention of the plating solution from being decomposed and so on.
Preferably, according to a fourth aspect of the present invention, the electroforming apparatus in the constitution as defined in the third aspect is characterized in that the anode part is formed as a titanium basket, and the electroforming apparatus further comprises a diaphragm and a shield plate respectively placed between the titanium basket and the object to-be-plated on the cathode part, a first piping part for feeding the plating solution to a space between the titanium basket, and the diaphragm and a second piping part for feeding the plating solution to a space between the cathode part and the shield plate.
According to the above constitution, since the electroforming apparatus further comprises the first piping part for feeding the plating solution to the space between the titanium basket and the diaphragm and the second piping part for feeding the plating solution to the space between the cathode part and the shield plate, the plating solution may be prevented from being retained, while the temperature of the plating solution may be prevented from increasing in the case of applying current, resulting in a more effective prevention of the plating solution from being decomposed.
Preferably, according to a fifth aspect of the present invention, the electroforming apparatus in the constitution as defined in the fourth aspect is characterized in that the titanium basket is formed at a distance of about 5 to 20 mm from the diaphragm, while the diaphragm is formed at a distance of about 10 to 30 mm from the shield plate.
According to the above constitution, since the titanium basket is formed at a distance of about 5 to 20 mm from the diaphragm, while the diaphragm is formed at a distance of about 10 to 30 mm from the shield plate, an increase in temperature of the plating solution resulting from the application of current may be controlled with accuracy, resulting in more effective prevention of the plating solution from being decomposed.
Preferably, according to a sixth aspect of the present invention, the electroforming apparatus in the constitution as defined in the fifth aspect is characterized in that the titanium basket is formed in the shape of a box as a whole, and at least the surface facing the cathode part out of the surfaces of the titanium basket comprises a meshed part.
Preferably, according to a seventh aspect of the present invention, the electroforming apparatus in the constitution as defined in the fourth aspect is characterized in that the shield plate has an opening in the center, while a outer periphery part having a plurality of holes is formed in the outer periphery of the opening.
According to the above constitution, since the shield plate has an opening in the center, while the periphery part having the plurality of punched holes is formed in the outer periphery of the opening, the current density of the cathode part may be increased largely without the need for increasing largely the current density of the anode part, while the object to be plated may be plated with a uniform coating thickness.
Preferably, according to a eighth the present invention, the electroforming apparatus in the constitution as defined in the first aspect is characterized in that the cathode part has a cap base to place the object to be plated and a cap to hold the object to be plated, and a ring-shaped member is placed between the object to be plated and the cap.
According to the above constitution, since the ring-shaped member is placed between the object to be plated and the cap, the contact area of the object to be plated with the ring-shaped member is increased to permit less breakage of the current-conductive coating formed on the object to be plated, resulting in less occurrence of defective conduction of current.